In wavelength division multiplexing (WDM) optical communication systems a single optical fiber may be used to carry multiple optical signals. The multiple optical signals are multiplexed to form a multiplexed signal or WDM signal with each of the multiple signals being modulated on separate channels. Each channel may be at an associated wavelength that is separated from adjacent channels by a defined channel-spacing, e.g. according to a channel plan established by the International Telecommunications Union (ITU). The range of wavelengths that may be transmitted on the system is known as the system bandwidth. Systems may utilize their system bandwidth to carry a desired number of channels with desired modulation format and bit rate.
The WDM signal may include data channels for communicating user data signals between terminals coupled to the system. In addition to data channels, a WDM signal may include one or more optical supervisory channels. Optical supervisory channels are not used to communicate user data between terminals, but instead carry supervisory signals used for monitoring the transmission path and/or the elements coupled to the transmission path. The supervisory signals may be at a relatively low power compared to the data signals, and may be modulated using On-Off-Keying (OOK). The supervisory signals may also have relatively low data rates, e.g. in the kb/s range, compared to the data signals, which may be in the Gb/s range. The supervisory channels are provided at a different wavelength than the data channels to limit signal interference. Supervisory channels may be provided at the edges of the system bandwidth. In some configurations, the frequency spacing between supervisory channels and the next adjacent data channel may be 100 GHz.
One challenge associated with long-haul optical transmission paths is polarization dependent loss (PDL) incurred by signals launched on the path. In general, different polarizations suffer different amounts of loss during transmission. To mitigate the effects of PDL, it is known to incorporate a polarization scrambler in a supervisory signal transmitter. As is known, a polarization scrambler rapidly varies the polarization of a signal so the average degree of polarization (DOP) over time is close to zero. FIG. 5, for example, illustrates a prior art supervisory signal transmitter wherein supervisory signal data is modulated on the output of a laser 502 by a supervisory signal data modulator 504. The output of the supervisory signal data modulator is polarization scrambled by a polarization scrambler 506, the output of which is provided as the supervisory signal on a supervisory channel.